Memory devices commonly store and retrieve information by manipulating characteristics of their cells. For example, chalcogenide glass phase-change memory utilizes thermal energy to switch the chalcogenide glass between amorphous and crystalline states. The amorphous and crystalline states have different electrical resistivities. Accordingly, information is stored or encoded to the chalcogenide glass phase-change memory by altering the state of the cells, and information is retrieved or decoded by determining the resitivities or resistances of the cells (e.g. by applying a voltage to the cells and measuring the resulting electrical current).
Different types of memory device cells store different amounts of information. Single level memory cell devices store one bit of information per a cell by varying between two states. For example, a single level flash memory cell has a floating gate that is capable of holding an electric charge. A cell having a charged floating gate illustratively represents a binary “0,” and a cell having an un-charged floating gate illustratively represents a binary “1.”
Multilevel memory cell devices store more than one bit of information per a cell by varying between three or more states. For example, a multilevel flash memory cell illustratively stores varying amounts electrical charge to its floating gate. The highest charged floating gate state may represent a binary “00,” and the lowest charged floating gate state may represent a binary “11.” Floating gate charges between the highest and lowest values illustratively represent a binary “01” and “10.”
Memory devices commonly have data retention issues. For example, properties of flash memory cells may change based on the number of write and erase cycles performed on the cells. One such illustrative property is the ability of the cells to be charged and to retain a charge. These changes may cause data to be incorrectly written to a cell or for a cell to lose data once it has been written.